If you purchased your device from Apple, you can request a receipt from the retail location.
Check your email for a copy of the receipt. Here’s how to locate the Apple Store and their contact information: Apple Store - Find a Store - Apple Apple Store - Find a Store - Apple
If your purchase was made at the Apple Online Store, please view the section titled “View or Print Invoice” here: Viewing & Changing Orders - Shopping Help - Apple Viewing & Changing Orders - Shopping Help - Apple
If your device was purchased from an Apple Authorized Reseller, it is recommend contacting them directly for a receipt.
Keep your receipt after you buy an Apple device
Keep your receipt after you buy an Apple device - Apple Support
If these are purchase through the App Store(?)—
View your purchase history for the App Store and other Apple media services - Apple Support
View your purchase history for the App Store, iTunes Store ...